1. Field of the Invention
This invention relates generally to rotary plug valves, and more particularly, but not by way of limitation, to rotary plug valves including replaceable valve body liners.
2. Description of the Prior Art
A variety of rotary plug valves have been developed and used heretofore, which valves include replaceable valve body liners interposed between the valve body and the valve plug. The liner protects the valve body against wear and erosion and provides a replaceable bearing surface adjacent the valve plug.
In some types of prior plug valves having a tapered plug, the relatively thin-walled valve bodies undergo expansion when the valve is closed and high line pressure is applied. The expansion of the valve body around the tapered plug permits high-pressure fluid to pass between the body and the valve plug. A force is then developed which acts across the tapered plug, tending to drive it down into the tapered interior of the valve body, and thus wedging the plug in the valve body. This wedging action which occurs as a result of the force differential acting across the tapered plug causes the opening torque to be excessive, and opening of the valve is rendered extremely difficult.
One solution for the undesirable "freezing" effect which thus occurs and prevents easy opening of the valve in high pressure service has been to increase the thickness of the valve body in which the tapered plug is located to thereby reduce the degree of expansion, and thus the extent of leakage of high line pressure fluid around the tapered core. It has also been proposed to inject a pressurized fluid into the valve body at a location where it will act against the small surface area at the small end of the tapered plug, and thus tend to restore a balance of pressure forces and oppose forcing of the tapered plug into the tapered cavity of the valve body.
More recently, it has been proposed in at least one commercially available valve, occasionally referred to as a "lo-torq" valve, to use a cylindrical valve plug or core which is surrounded at its outer cylindrical surface by two tapered liner inserts which are, considered together, in the form of a frusto-conical insert having a cylindrical bore through the center thereof, and split into two halves along a plane passed through the central axis of such frusto-conical insert. The cylindrical inside diameter of the two insert halves corresponds to the cylindrical outside diameter of the cylindrical plug. Again, however, in this type of valve, pressure enlargement of the body cavity within the valve body, accompanied by floating downstream of the composite plug and liner (insert) parts, permits the valve cavity to become filled with high-pressure fluid leaking past the upstream seal, and again causing wedging of the two liner or insert parts downwardly in the valve body, and against the outer peripheral surface of the plug. As the tapered inserts are driven further into the body cavity, they act as wedges and tighten and clamp upon the plug, making it very difficult to open the valve without damaging the parts.
To offset this differential force wedging effect, it is sometimes proposed to balance the pressure system, as previously described, by the injection of a pressurized fluid into the lower side of the body cavity, and against the relatively small lower end face of the two frusto-conically shaped inserts. Some efforts have also been recently made to reduce the propensity of the plug and insert combination to float downstream under high pressure in valves of this type by providing grooves or recesses in the bonnet of the valve into which the two split insert halves are keyed or fitted. This arrangement undertakes to prevent, by such physical engagement, the plug and inserts drifting downstream when the valve body enlarges and the plug and inserts are otherwise free to float. While this has effected some reduction in the floating action described, and the following wedging action by which the inserts are driven into clamping engagement with the plug, the latter difficulty has not been entirely alleviated by this expedient. Valves of the sort described above are shown in McGuire et al. U.S. Pat. No. 3,133,722 and Stogner U.S. Pat. No. 2,813,695.
A more recent effort to obtain the best compromise between optimum sealing of the plug valve and low opening torque at high pressure is reflected in the plug valve shown in Thompson et al. U.S. Pat. No. 3,346,002. The Thompson et al. valve has several advantages over the tapered plug type valves, or the tapered, generally frusto-conical split insert type valves hereinbefore described. In the Thompson et al. valve, both the plug, and an insert used in conjunction with the plug to provide a seat for the plug and concomitantly seal with the valve body, are cylindrical in configuration. This makes the valve considerably more economical to fabricate as the cylindrical surfaces are much less expensive to generate during construction than tapered plugs or inserts. Moreover, the Thompson et al. valve undertakes to afford at least a partial solution to the force differential problem previously described, and indicated in the foregoing discussion to result from the difference in cross-sectional areas characteristic of plugs or inserts which are tapered to a reduced diameter at their lower end.
In the Thompson et al. valve, the cylindrical plug which is provided is seated within a split cylindrical liner or sleeve which is sealingly positioned within the valve body cavity. The sleeve or liner is split at one side thereof so that it can expand or contract, and is machined to closely and sealingly engage the cylindrical external periphery of the valve plug which is positioned within the split liner. Despite the split along one side of the cylindrical liner, it functions under high pressure as an essentially unitary rigid member. The high-pressure fluid acting on the upstream side of the line, within the annular area circumscribed by the O-ring between the liner and the inside of the valve body cavity, forces that area of the liner against the valve plug, thereby clamping the valve plug, and causing a relatively high amount of torque to be required to open the valve when such high-pressure conditions exist. Due to its unitary rigid design, the liner member cannot separate from the valve plug to relieve this clamping, vise-like contact with the valve plug. High-pressure fluid cannot force its way between the liner member and the valve plug.
Another valve similar in concept to that of Thompson et al. is that shown in Foster U.S. Pat. No. 3,423,067.
Another effort to achieve a low opening torque valve by the use of a cylindrical liner and plug in a cylindrical bore is that shown in Hamer U.S. Pat. No. 3,061,267. In the Hamer valve, the cylindrical plug is seated within a four-piece cylindrical liner which is sealingly positioned within the valve body cavity. Between the outer diameter of the liner and the inner diameter of the valve body cavity are positioned resilient members which, when axially compressed by means of a keeper ring which screws into the top of the valve body, expand radially inwardly and outwardly to urge the liner members into intimate contact with the valve plug. Hamer recognized that a lower opening torque can be achieved at high pressures by not having a fluid seal on the upstream side of the valve plug and relying on only the downstream seal, but in order to achieve this, Hamer found it necessary to use a sealing means between the liner and the valve body cavity which was a one-way sealing means which could seal only on the downstream side of the valve and never on the upstream side of the valve. Hamer's valve, therefore, always has only a downstream seal, even at very low pressures. The problems with high opening torque apply only at relatively high pressures. There are advantages to having an upstream seal at lower pressures and at such lower pressures, opening torque is not a problem. Hamer found it necessary, nevertheless, to sacrifice the benefits of having both an upstream seal and a downstream seal at lower pressures in order to achieve the benefit of having only a downstream seal at the higher pressures.
The valve of Bishop U.S. Pat. No. 3,178,152 uses elastic retaining rings, but that valve is completely non-analogous to the valves discussed above, and to the valve of this invention. Bishop's valve is for very low pressure service only. It uses retaining rings to hold a casing in contact with a plug. It has no liner between the casing and the plug.